Scrambling is a method for line coding, widely used in standard optical, wireline and wireless communication systems. Scrambled data patterns typically feature approximately equal number of logic “ones” and “zeros”, thus ensuring proper biasing of the line drivers and receivers. Additionally, scrambled line code maintains sufficient rate of bit transitions necessary to keep the receiver in lock and enable reliable clock recovery.
The three scrambling methods used in various communication standards are Frame-Synchronous Scrambling (FSS), Self-Synchronous Scrambling (SSS) and Distributed Sample Scrambling [ref]. Frame-synchronous scrambling (FSS), to which the present disclosure is directed, is used in major data communications standards such as the family of Synchronous Optical NETwork (SONET) standards published by American National Standards Institute (ANSI) and International Telecommunications Union (ITU), Broadband Passive Optical Networks (BPON) ITU-T G.983, Gigabit-capable Passive Optical Networks (GPON) ITU-T G.984, Wireless Local Area Network (WLAN) IEEE 802.11 etc. Frame-synchronous scrambling is implemented in the transmitter unit by performing the bitwise exclusive XOR operation between the outgoing data bit stream and the cyclically repeatable scrambler sequence. FSS is popular because its implementation is simple and because bit errors in the received signal are not multiplied by the FSS descrambler. However, FSS is susceptible to “pathological” bit patterns, i.e. undesirable bit patterns that happen to match the scrambler sequence, which can result in long sequences of all-zero or all-one bits in the scrambled signal. Transmission of such sequences causes clock and data recovery failure and unlocking of the receiver(s), leading to heavy packet loss. The pathological patterns may either be created accidentally, or deliberately by a malicious attacker aiming to impair the network traffic. The probability of occurrence of pathological data patterns is higher for shorter scrambler sequences.
This issue has become particularly critical in the emerging access technology known as passive optical networks (PON), due to their high exposure to individual users. Particularly affected are PON standards BPON and GPON standards which utilize scrambler sequences of only 127 bits. To compensate for the standard's vulnerability to attack due to the described FSS weakness, the ITU-T standard G.984 (GPON) recommends the use of encryption of all downstream unicast traffic. However, this measure provides only partial protection from attacks since it is not applicable to GPON multicast traffic, which cannot be encrypted.
What is required is a system and method for protecting communications against pathological bit patterns in a communications system using frame-synchronous scrambling.